This invention relates generally to phase detectors suitable for use for detecting a mutual phase difference between two microwave signals, and more particularly, to an improved phase detector comprising a field effect transistor and being able to detect a mutual phase difference between two microwave signals with high sensitivity and superior stability.
For detecting a mutual phase difference between two microwave signals, there has been already proposed a phase detector which is composed of a pair of input signal paths, a quarter wavelength hybrid ring which has four branch signal paths each selected in length substantially to correspond to a quarter of the wavelength of an input signal and interconnected to form a rectangular signal path with a pair of input ends connected to the input signal paths respectively and a pair of output ends, and a pair of diodes connected to the output ends of the quarter wavelength hybrid ring, respectively, in the respective directions opposite to each other. In such a phase detector, first and second microwave signals having a mutual phase difference therebetween are supplied to the input ends of the quarter wavelength hybrid ring through the input signal paths, respectively, and transmitted to the output ends of the quarter wavelength hybrid ring through the rectangular signal path therein. The microwave signals appearing at the output ends of the quarter wavelength hybrid ring are supplied to the diodes connected thereto in the opposite directions, respectively. The diodes detect the supplied microwave signals and cooperate with each other to combine their detected outputs so as to produce an output signal representing the mutual phase difference between the first and second microwave signals supplied to the input signal paths. Thus, the mutual phase difference between the first and second microwave signals is detected.
In the previously proposed phase detector as above mentioned, a portion for detecting substantially a mutual phase difference between two input microwave signals is composed with a pair of diodes as described above. Consequently, with the previously proposed phase detector, it is quite difficult to increase the sensitivity of the phase detecting operation. Further, a reflection characteristic in relation to an input microwave signal at each of the diodes and a temperature characteristic in relation to the DC detected output and an input microwave signal at each of the diodes may vary depending on the power of the input signal. It is also very difficult to make such variations in the characteristics identical between both of the diodes used in a pair, and therefore the previously proposed phase detector tends to be lacking in stability in detecting operation.